As an existing position detecting device, as Japanese Unexamined Patent Application Publication No. 2006-178362, A discloses, it is known that an optical device which holds a lens and is provided with a movable frame being movable along the optical axis of the lens is configured so that a printed circuit board being printed with a coil is arranged in parallel to the optical axis and a planar conductive member is arranged in a manner to confront against the printed circuit board. The conductive member is formed into a trapezoidal shape so that a width of the confronting surface thereof against the coil in the direction orthogonal to the optical axis direction can be changed in accordance with a position thereof with respect to the optical axis direction, thereby an inductance of the coil can be changed as the lens moves. According to the change of the inductance, a position of the lens can also be detected.